Forty Seven (FTSV) 10-Q 12 Nov 19 2019 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Nov. 05, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Trading Symbol	FTSV
Entity Registrant Name	Forty Seven, Inc.
Entity Central Index Key	0001667633
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		42,276,130
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Incorporation, State or Country Code	DE
Entity File Number	001-38554
Entity Tax Identification Number	47-4065674
Entity Address, Address Line One	1490 O’Brien Drive
Entity Address, Address Line Two	Suite A
Entity Address, City or Town	Menlo Park
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94025
City Area Code	650
Local Phone Number	352-4150
Document Quarterly Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, $0.0001 par value
Security Exchange Name	NASDAQ

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents		$ 31,783	$ 10,837	[1]
Short-term investments		134,959	128,186	[1]
Prepaid expenses and other current assets		10,583	6,835	[1]
Total current assets		177,325	145,858	[1]
Property and equipment, net		1,315	1,360	[1]
Operating lease right-of-use assets		2,282
Other assets		2,661	2,219	[1]
Total assets		183,583	149,437	[1]
Current liabilities:
Accounts payable		3,164	4,621	[1]
Accrued liabilities		16,519	9,044	[1]
Lease liabilities, current		1,434
Deferred grant funding, current		2,585	1,744	[1]
Total current liabilities		23,702	15,409	[1]
Lease liabilities, noncurrent		1,228
Deferred rent, noncurrent	[1]		331
Other long-term liabilities		323	476	[1]
Total liabilities		25,253	16,216	[1]
Commitments and Contingencies				[1]
Stockholders’ equity:
Common stock		4	3	[1]
Additional paid-in capital		359,390	273,069	[1]
Accumulated other comprehensive income (loss)		90	(82)	[1]
Accumulated deficit		(201,154)	(139,769)	[1]
Total stockholders’ equity		158,330	133,221	[1]
Total liabilities and stockholders’ equity		$ 183,583	$ 149,437	[1]
[1]	The balance sheet as of December 31, 2018 is derived from the audited financial statements as of that date.

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
License revenue	$ 15,678		$ 15,678
Revenue from Contract with Customer, Product and Service [Extensible List]	us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 27,074	$ 17,984	$ 65,029	$ 42,733
General and administrative	4,977	4,383	14,618	11,588
Total operating expenses	32,051	22,367	79,647	54,321
Loss from operations	(16,373)	(22,367)	(63,969)	(54,321)
Interest and other income, net	1,210	708	2,584	1,165
Net loss	(15,163)	(21,659)	(61,385)	(53,156)
Unrealized gains (losses) on available-for-sale securities	31	(4)	172	12
Comprehensive loss	$ (15,132)	$ (21,663)	$ (61,213)	$ (53,144)
Net loss per share, basic and diluted	$ (0.38)	$ (0.71)	$ (1.80)	$ (3.63)
Shares used in computing net loss per share, basic and diluted	39,772,452	30,430,898	34,129,449	14,643,348

Condensed Statements of Convert

Condensed Statements of Convertible Preferred Stock and Stockholders' Equity - USD ($) $ in Thousands	Total		Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ 83,462		$ 149,397	$ 1	$ 3,507	$ (44)	$ (69,399)
Beginning balance, shares at Dec. 31, 2017			16,215,896	6,751,157
Settlement of fractional shares from reverse stock split, shares				(15)
Vesting of early exercised stock options	13				13
Stock-based compensation	431				431
Repurchase of common stock, shares				(41,935)
Net loss	(14,775)						(14,775)
Other comprehensive income (loss)	(27)					(27)
Ending balance at Mar. 31, 2018	69,104		$ 149,397	$ 1	3,951	(71)	(84,174)
Ending balance, shares at Mar. 31, 2018			16,215,896	6,709,207
Beginning balance at Dec. 31, 2017	83,462		$ 149,397	$ 1	3,507	(44)	(69,399)
Beginning balance, shares at Dec. 31, 2017			16,215,896	6,751,157
Net loss	(53,156)
Other comprehensive income (loss)	12
Ending balance at Sep. 30, 2018	149,235			$ 3	271,819	(32)	(122,555)
Ending balance, shares at Sep. 30, 2018				31,048,089
Beginning balance at Mar. 31, 2018	69,104		$ 149,397	$ 1	3,951	(71)	(84,174)
Beginning balance, shares at Mar. 31, 2018			16,215,896	6,709,207
Issuance of common stock for exercise of stock options	55				55
Issuance of common stock for exercise of stock options, shares				28,630
Vesting of early exercised stock options	171				171
Stock-based compensation	616				616
Net loss	(16,722)						(16,722)
Other comprehensive income (loss)	43					43
Ending balance at Jun. 30, 2018	53,267		$ 149,397	$ 1	4,793	(28)	(100,896)
Ending balance, shares at Jun. 30, 2018			16,215,896	6,737,837
Issuance of common stock upon initial public offering, net of issuance costs	116,336			$ 1	116,335
Issuance of common stock upon initial public offering, net of issuance costs, shares				8,090,250
Preferred stock conversion to common stock			$ (149,397)	$ 1	149,396
Preferred stock conversion to common stock, shares			(16,215,896)	16,215,896
Issuance of common stock for exercise of stock options	11				11
Issuance of common stock for exercise of stock options, shares				4,106
Vesting of early exercised stock options	13				13
Stock-based compensation	1,271				1,271
Net loss	(21,659)						(21,659)
Other comprehensive income (loss)	(4)					(4)
Ending balance at Sep. 30, 2018	149,235			$ 3	271,819	(32)	(122,555)
Ending balance, shares at Sep. 30, 2018				31,048,089
Beginning balance at Dec. 31, 2018	133,221	[1]		$ 3	273,069	(82)	(139,769)
Beginning balance, shares at Dec. 31, 2018				31,079,150
Issuance of common stock for exercise of stock options	789				789
Issuance of common stock for exercise of stock options, shares				174,793
Issuance of common stock pursuant to the ESPP	588				588
Issuance of common stock pursuant to the ESPP, shares				44,656
Vesting of early exercised stock options	113				113
Stock-based compensation	1,104				1,104
Net loss	(23,016)						(23,016)
Other comprehensive income (loss)	100					100
Ending balance at Mar. 31, 2019	112,899			$ 3	275,663	18	(162,785)
Ending balance, shares at Mar. 31, 2019				31,298,599
Beginning balance at Dec. 31, 2018	133,221	[1]		$ 3	273,069	(82)	(139,769)
Beginning balance, shares at Dec. 31, 2018				31,079,150
Net loss	(61,385)
Other comprehensive income (loss)	172
Ending balance at Sep. 30, 2019	158,330			$ 4	359,390	90	(201,154)
Ending balance, shares at Sep. 30, 2019				42,271,052
Beginning balance at Mar. 31, 2019	112,899			$ 3	275,663	18	(162,785)
Beginning balance, shares at Mar. 31, 2019				31,298,599
Issuance of common stock for exercise of stock options	612				612
Issuance of common stock for exercise of stock options, shares				133,723
Vesting of early exercised stock options	13				13
Stock-based compensation	1,125				1,125
Net loss	(23,206)						(23,206)
Other comprehensive income (loss)	41					41
Ending balance at Jun. 30, 2019	91,484			$ 3	277,413	59	(185,991)
Ending balance, shares at Jun. 30, 2019				31,432,322
Issuance of common stock in public offering, net of issuance costs	80,461			$ 1	80,460
Issuance of common stock in public offering, net of issuance costs, shares				10,781,250
Issuance of common stock for exercise of stock options	24				24
Issuance of common stock for exercise of stock options, shares				7,568
Issuance of common stock pursuant to the ESPP	272				272
Issuance of common stock pursuant to the ESPP, shares				49,912
Vesting of early exercised stock options	13				13
Stock-based compensation	1,208				1,208
Net loss	(15,163)						(15,163)
Other comprehensive income (loss)	31					31
Ending balance at Sep. 30, 2019	$ 158,330			$ 4	$ 359,390	$ 90	$ (201,154)
Ending balance, shares at Sep. 30, 2019				42,271,052
[1]	The balance sheet as of December 31, 2018 is derived from the audited financial statements as of that date.

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (61,385)	$ (53,156)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	3,437	2,318
Depreciation and amortization	386	289
Amortization of right-of-use assets	759	0
Accretion of discounts on marketable securities	(977)	(293)
Realized gain on sale of available-for-sale securities	(6)	0
Change in fair value of embedded derivative	(13)	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,748)	(2,172)
Other assets	(445)	69
Accounts payable	(1,469)	185
Accrued liabilities	7,631	554
Deferred grant funding	841	1,375
Lease related liabilities	(866)	(86)
Other long-term liabilities	0	(1)
Net cash used in operating activities	(55,855)	(50,918)
Cash flows from investing activities:
Purchases of property and equipment	(328)	(106)
Purchases of available-for-sale securities	(166,040)	(138,071)
Proceeds from sales of available-for-sale securities	3,996	0
Proceeds from maturities of available-for-sale securities	156,427	71,478
Net cash used in investing activities	(5,945)	(66,699)
Cash flows from financing activities:
Proceeds from issuance of common stock upon ESPP purchase	859	0
Proceeds from issuance of common stock upon exercise of stock options	1,426	166
Proceeds from public offering of common stock, net of issuance costs	80,461	116,460
Net cash provided by financing activities	82,746	116,626
Net increase (decrease) in cash and cash equivalents	20,946	(991)
Cash and cash equivalents — beginning of period	10,837	24,417
Cash and cash equivalents — end of period	31,783	23,426
Supplemental disclosures of cash flow information:
Operating cash flows paid for operating leases	1,015	0
Noncash investing and financing activities:
Purchases of property and equipment included in accounts payable	13	0
Lease liability obtained in exchange for right-of-use asset	712	0
Conversion of convertible preferred stock to common stock at close of initial public offering	0	149,397
Deferred offering costs included in accounts payable and accrued liabilities	$ 0	$ 124

Description of Business

Description of Business	9 Months Ended
	Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Basis of Presentation	1. Description of Business The Company is a clinical-stage immuno-oncology company focused on developing novel checkpoint therapies to activate macrophages in the fight against cancer. Forty Seven was founded based on the insight that blocking CD47, a key signaling molecule that is over-expressed on cancer cells, renders tumors susceptible to macrophages and the innate immune system. By harnessing macrophages, the Company believes that its lead product candidate, magrolimab (formerly known as 5F9), dosed as a monotherapy and in combination with marketed cancer therapies, can transform the treatment of cancer. Liquidity In the course of its development activities, the Company has sustained operating losses and expects to continue to generate operating losses for the foreseeable future. The Company’s ultimate success depends on the outcome of its research and development activities. The Company had cash, cash equivalents and short-term investments of $166.7 million as of September 30, 2019. Since inception through September 30, 2019, the Company has incurred cumulative net losses of $201.2 million. Management expects to incur additional losses in the future to conduct product research and development and recognizes the need to raise additional capital to fully implement its business plan. The Company intends to raise such capital through the issuance of additional equity financing and/or third-party collaboration funding. However, if such financing is not available at adequate levels, the Company will need to reevaluate its operating plan and may be required to delay the development of its products. The Company expects that its cash, cash equivalents and short-term investments will be sufficient to fund operating expenses and capital expenditure requirements for a period of at least one year from the date these interim condensed financial statements are filed with the Securities and Exchange Commission (“SEC”). In July 2019, the Company filed a Registration Statement on Form S-3, as amended (File No. 333-232498), declared effective by the SEC on July 12, 2019 (the “Shelf Registration Statement”), related to the offering of up to $250.0 million of common stock, preferred stock, debt securities and warrants. The Company may seek to use the Shelf Registration Statement at any time or from time to time to offer, in one or more offerings, . SM Cantor Fitzgerald & Co. In July 2019, pursuant to the Shelf Registration Statement, the Company completed an underwritten public offering of 10,781,250 shares of the Company’s common stock, including 1,406,250 shares sold pursuant to the underwriters’ exercise of their option to purchase additional shares at a public offering price of $8.00 per share. The net proceeds from the offering to the Company were $80.5 million.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The interim condensed financial statements are unaudited. The unaudited interim condensed financial statements have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s financial position, results of operations and cash flows for the periods presented. The results of operations for the three and nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any other future annual or interim period. The condensed balance sheet as of December 31, 2018 included herein was derived from the audited financial statements as of that date. These condensed financial statements should be read in conjunction with the Company's audited financial statements and related notes as set forth in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018, as filed with the SEC on March 28, 2019. Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenue and expenses during the reporting period. Significant estimates and assumptions made in the accompanying financial statements include but are not limited to revenue recognition, the fair value of stock options, the fair value of investments, income tax uncertainties, lease liability, and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates. Revenue Recognition License and Collaboration Arrangements The Company accounts for revenue recognition under Accounting Standards Codification (“ASC”), Revenue from Contracts with Customers (Topic 606) Under ASC 606, the Company recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services. The Company enters into collaborative arrangements with partners that fall under the scope of ASC, Collaborative Arrangements (Topic 808) In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues attributed to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not generally considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of any development milestones, and if necessary, adjusts its estimate of the transaction price. Any such adjustments would be recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. Foreign Currency Transactions Gains or losses from foreign currency transactions are included in interest and other income, net, in the statements of operations and comprehensive loss. The Company recognized $0.3 million in foreign currency gains for the three months and nine months ended September 30, 2019. Foreign currency losses for the three months ended September 30, 2018 were immaterial. The Company recognized $0.2 million in foreign currency losses for the nine months ended September 30, 2018. Fair Value Measurement Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active; Level 3— Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. Leases The Company adopted Accounting Standards Update No. 2016-02, Leases Rent expense for the operating leases is recognized on a straight-line basis over the lease term and is included in operating expenses on the statements of operations and comprehensive loss. Variable lease payments include lease operating expenses. The accompanying condensed financial statements as of and for the three and nine months ended September 30, 2019 are presented under ASC 842. The prior periods continue to be reported in accordance with previous lease guidance, ASC 840, Leases The impact of the adoption of ASC 842 on the accompanying condensed balance sheet as of January 1, 2019 was as follows: December 31, 2018 Adjustments due to the adoption of ASC 842 January 1, 2019 (In thousands) Assets Operating lease right-of-use asset $ — $ 2,328 $ 2,328 Liabilities and stockholders' equity Deferred rent classified as accrued liabilities $ 155 $ (155 ) $ — Lease liability, current $ — $ 968 $ 968 Lease liability, noncurrent $ — $ 1,847 $ 1,847 Deferred rent, noncurrent $ 331 $ (331 ) $ — The adjustments due to the adoption of ASC 842 related to the recognition of an operating lease right-of-use asset and lease liability for the Company's existing property operating lease and the derecognition of the deferred rent recognized under ASC 840. There was no impact on the Company’s statement of operations and comprehensive loss from the adoption and no cumulative-effect adjustment to the beginning accumulated deficit.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The Company measures and records its cash equivalents and short-term investments at fair value. Money market funds and US government debt securities are measured at fair value on a recurring basis using quoted prices and are classified as a Level 1 input. Short-term investments are measured at fair value based on inputs other than quoted prices that are derived from observable market data and are classified as Level 2 inputs. There were no transfers between Levels 1, 2 or 3 for any of the periods presented. All of the investments held as of September 30, 2019 and December 31, 2018 had maturities of less than one year. There were no significant realized gains or losses on investments for the three and nine months ended September 30, 2019 and 2018. The fair value and amortized cost of cash equivalents and available-for-sale securities by major security type as of September 30, 2019 and December 31, 2018 are presented in the following tables: As of September 30, 2019 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Market Value (In thousands) Money market funds Level 1 $ 30,546 $ — $ — $ 30,546 US government debt securities Level 1 28,039 13 (10 ) 28,042 Commercial paper Level 2 29,612 — — 29,612 Corporate debt securities Level 2 52,167 66 — 52,233 Asset-backed securities Level 2 25,051 22 (1 ) 25,072 Total cash equivalents and available-for-sale securities $ 165,415 $ 101 $ (11 ) $ 165,505 As of December 31, 2018 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Market Value (In thousands) Money market funds Level 1 $ 7,959 $ — $ — $ 7,959 US government debt securities Level 1 15,963 — (1 ) 15,962 Commercial paper Level 2 43,277 — — 43,277 Corporate debt securities Level 2 46,186 — (54 ) 46,132 Asset-backed securities Level 2 22,842 — (27 ) 22,815 Total cash equivalents and available-for-sale securities $ 136,227 $ — $ (82 ) $ 136,145 The Company’s contingent milestone payments in its agreement with the Leukemia & Lymphoma Society, Inc. (“LLS”) were concluded to be an embedded derivative. The embedded derivative contains unobservable inputs that are supported by little or no market activity at the measurement date. Accordingly, the Company’s embedded derivative is measured at fair value on a recurring basis using unobservable inputs that are classified as Level 3 inputs. The Company recorded a liability for the derivative of approximately $0.3 million, as part of other long-term liabilities as of September 30, 2019 and December 31, 2018. Refer to Note 5 for the valuation techniques and assumptions used in estimating the fair value of the embedded derivative. The change in fair value of the embedded derivative is presented in the following table: Nine Months Ended September 30, 2019 (In thousands) Beginning balance $ 331 Change in fair value of embedded derivative (13 ) Ending balance $ 318

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	4. Balance Sheet Components Accrued Liabilities Accrued liabilities consist of the following: September 30, 2019 December 31, 2018 (In thousands) Accrued research and development expenses $ 12,605 $ 5,870 Accrued bonuses 2,119 1,602 Deferred rent, current — 155 Other 1,795 1,417 Total accrued liabilities $ 16,519 $ 9,044

Research and License Agreements

Research and License Agreements	9 Months Ended
	Sep. 30, 2019
Research And License Agreements [Abstract]
Research and License Agreements	5. Research and License Agreements California Institute of Regenerative Medicine Grants In J u r 1 t m n a w d a research grant I T R s t u t v m l s e - b s a y m n w t h t a w 0 ov r f y rs As of September 30, 2019, and December 31, 2018, the Company had received $9.2 million and $7.2 million under the award, respectively In November 2017, the Company was awarded a second research grant from CIRM for a separate clinical trial study. The total amount of the research grant awarded was $5.0 million in various milestone-based payments over a period of five years. During 2018, the award was amended to $3.2 million in various-milestone payments over a period of five years, as was provided for under the terms of the original award because the Company opted not to expand the patient population participating in the study. In July 2019, the award was further amended to include the patient population expansion that the Company previously opted out of in 2018. In connection with the same, the award was reinstated to $5.0 million consistent with the original award amount. During the three and nine months ended September 30, 2019, the Company received $0.5 million and $1.2 million related to this grant, respectively. As of September 30, 2019, and December 31, 2018, the Company had received $3.6 million and $2.4 million under the award, respectively t C m a n is obligated t a o y a l i n li c e s i f e a s o e s a l RM - f n e product candidates I M - f n e e c n l g y Leukemia & Lymphoma Society Grant In In July 2019, the Company entered into an amendment to its agreement with LLS to further advance the treatment of myelodysplastic syndromes (“MDS”). Under the amendment, the Company is eligible for up to $3.0 million in additional grant funding based on milestone payments from LLS upon the achievement of certain clinical or regulatory milestones in addition to the $4.2 million award that the Company has received pursuant to the March 2017 agreement, as amended. Pursuant to the amendment, the Company could be required in the future to pay amounts to LLS upon reaching certain development and regulatory approval milestones on the additional funding, up to a maximum of $6.0 million in the aggregate. The Company concluded that the contingent milestone payments under the amendment were an embedded derivative. As of September 30, 2019, the Company had not received any funding related to this grant and therefore the Company did not record any liability related to this derivative as of September 30, 2019. The Company recognizes research grants as a reduction of research and development expense when the eligible costs are incurred. Under both CIRM and LLS grants, the Company recognized a total of $0.6 million and $2.2 million as a reduction to research and development expenses for research grants for the three months ended September 30, 2019 and 2018, respectively. The Company recognized a total of $2.6 million and $5.6 million as a reduction to research and development expenses for research grants for the nine months ended September 30, 2019 and 2018, respectively. Other Collaboration Agreements Master Combination Study Agreement with Genentech, Inc. In November 2017, the Company entered into a master clinical trial collaboration agreement with Genentech to evaluate the safety, tolerability and preliminary efficacy of magrolimab combined with Genentech’s cancer immunotherapy, atezolizumab, a fully humanized monoclonal antibody targeting PD-L1, in two separate Phase 1b clinical trials (in patients with bladder cancer and AML, respectively). Pursuant to the agreement, the Company will supply magrolimab for the studies and will partially reimburse Genentech for its costs in connection with the bladder cancer study, and Genentech will supply atezolizumab for the studies and be solely responsible for all of its costs in connection with the AML study. Merck Collaboration Agreement In bluebird bio, Inc. Research Collaboration Agreement In September 2019, the Company entered into a research collaboration with bluebird bio, Inc. (“bluebird”) to pursue clinical proof-of-concept for the Company’s novel antibody-based conditioning regimen, FSI-174 (anti-cKIT antibody) plus magrolimab (anti-CD47 antibody), with bluebird’s ex vivo lentiviral vector hematopoietic stem cell (LVV HSC) gene therapy platform. This collaboration will focus initially on diseases that have the potential to be corrected with transplantation of autologous gene-modified blood-forming stem cells. As of September 30, 2019, the Company recorded a total receivable of $1.0 million from these collaborations for reimbursement of research and development costs incurred. Reimbursement under these collaboration agreements is recorded as a reduction to research and development expense. For the three months ended September 30, 2019 and 2018, the Company recognized $1.0 million and $0.4 million, respectively, as a reduction to research and development expenses under these collaboration agreements. For the nine months ended September 30, 2019 and 2018, the Company recognized $1.9 million and $1.0 million, respectively, as a reduction to research and development expenses under these collaboration agreements. Ono License and Collaboration Agreement In July 2019, the Company entered into an exclusive license and collaboration agreement with Ono Pharmaceutical Co., Ltd., (“Ono”). Under the agreement, the Company granted Ono an exclusive license to develop, manufacture and commercialize magrolimab, the Company’s monoclonal antibody against CD47, as well as other anti-CD47 antibodies controlled by the Company in Japan, South Korea, Taiwan and the ASEAN countries (the “Ono Territory”). The Company retains all rights to magrolimab and other licensed antibodies outside of the Ono Territory. Under the agreement, the parties will collaborate on the development, manufacturing and commercialization of magrolimab and other licensed antibodies. Each party will be responsible for conducting development and commercialization of licensed antibodies in its respective territory at its own cost. Further, each party will have the right to participate, at its cost, in global clinical studies of magrolimab and other licensed antibodies conducted by the other party. The Company received a one-time upfront nonrefundable payment from Ono of 1.7 billion Japanese Yen ($15.7 million US Dollars based on the exchange rate as of the date of the agreement) and is eligible to receive up to an additional 11.2 billion Japanese Yen if specified future development and commercial milestones are achieved by Ono. The Company is also eligible to receive tiered percentage royalties spanning from the mid-teens to the low-twenties on future net sales of magrolimab and other licensed antibodies in the Ono Territory, subject to certain offsets. The Company accounts for the Ono Agreement as a collaboration arrangement under ASC 808. The Company concluded that the license delivered to Ono is a distinct unit of account for which there is a vendor-customer relationship. For the license, the Company analogized to ASC 606 for the recognition, measurement and reporting of this unit of account. The up-front payment, future milestones and royalties are attributed to the license component of the Ono Agreement. For the cost-sharing and reimbursement-based activities, the Company will follow the presentation and disclosure guidance of ASC 808. As of September 30, 2019, the cost-sharing and reimbursement-based activities had not commenced under the agreement. The transaction price at inception included upfront fixed nonrefundable consideration of $15.7 million. All potential future milestones and other payments were considered constrained at the inception of the Ono Agreement. Revenue for the license of $15.7 million was recognized upon delivery of the license in July 2019.

Related-Party Relationship with

Related-Party Relationship with Stanford University	9 Months Ended
	Sep. 30, 2019
Related Party Transactions [Abstract]
Related-Party Relationship with Stanford University	6. Related-Party Relationship with Stanford University In November 2015, the Company entered into a technology license agreement with Stanford University, under which Stanford granted the Company exclusive licenses under certain patents and other intellectual property rights relating to our current product candidates, including magrolimab, and non-exclusive licenses under certain other patents and other intellectual property rights to develop, manufacture and commercialize products for use in certain licensed fields, including oncology. Under the agreement, the Company is required to pay Stanford a minimum annual fee, milestone payments upon the achievement of certain specified events and a royalty of a tiered single digit percentage on net sales of licensed products, reimburse patent-related expenses and share any non-royalty sublicensing income received related to the licensed technology. As a result of the $15.7 million license payment received under the Ono License and Collaboration Agreement (see Note 5), the Company recorded an estimated sublicensing fee liability of $0.3 million due to Stanford under the agreement during the three and nine months ended September 30, 2019. Dr. Weissman and Dr. Majeti, co-founders and members of the Company’s board of directors, are professors at Stanford. While employed by Stanford, Dr. Weissman and Dr. Majeti were co-inventors of some of the patents that the Company licenses under the agreement. Under Stanford's policies, as co-inventors Dr. Weissman and Dr. Majeti are entitled to receive a share of any royalties that the Company pays to Stanford under the agreement with respect to such patents. No royalty payments have been made to date. In addition, under Stanford’s policies, as co-inventors Dr. Weissman and Dr. Majeti are entitled to receive a share of the annual license fees that the Company pays to Stanford with respect to such patents.

Leases

Leases	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Leases	7. Leases The Company leases office property and laboratory space at its headquarters in Menlo Park (the “Menlo Park Lease”) through August 2021. The lease requires monthly lease payments subject to annual increases throughout the lease term and includes a renewal option at the election of the Company to extend the lease for an additional five years. The landlord provided the Company with a tenant improvement allowance of $646,000. In April 2019, the Company entered into a sublease (the “Sublease”) to obtain 6,230 rentable square feet to expand its current headquarters through February 2021. The lease requires monthly lease payments subject to annual increases throughout the lease term. In April 2019, the Company also entered into an amendment of the Menlo Park Lease to add additional space to its headquarter lease upon the expiration of the Sublease in February 2021. Under the terms of the lease amendment, the lease for the additional space will commence on March 1, 2021 and the base rent for the additional space will be approximately $36,000 per month. As of September 30, 2019, the weighted average remaining lease term was 1.8 years and the weighted average incremental borrowing rate used to determine the operating lease liabilities was 7.0%. The undiscounted future non-cancellable lease payments under the Company's operating leases are as follows: September 30, 2019 (In thousands) Years Remaining 2019 $ 392 2020 1,584 2021 864 Total undiscounted lease payments 2,840 Present value adjustment for minimum lease commitments (178 ) Lease liabilities $ 2,662 As of September 30, 2019, the current and noncurrent portion of the lease liabilities was $1.4 million and $1.2 million. Rent expense for the operating leases was $0.4 million and $0.2 million for the three months ended September 30, 2019 and 2018, respectively. Rent expense for the operating leases was $0.9 million and $0.8 million for the nine months ended September 30, 2019 and 2018, respectively. Variable lease payments for operating expenses were $0.2 million and $0.1 million for the three months ended September 30, 2019 and 2018, respectively. Variable lease payments for operating expenses were $0.4 million for the nine months ended September 30, 2019 and 2018.

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
Stockholders' Equity	8 . Stockholders’ Equity 2015 and 2018 Equity Incentive Plans The following summarizes option activity for the nine months ended September 30, 2019: Shares Issuable Under Options Weighted- Average Exercise Price Weighted- Average Remaining Contract Term Aggregate Intrinsic Value (In years) (In thousands) Balance, December 31, 2018 3,404,847 $ 6.55 8.82 $ 31,388 Options granted 233,652 13.55 Options exercised (316,084 ) 4.51 Options forfeited (114,612 ) 7.86 Balance outstanding September 30, 2019 3,207,803 $ 7.21 8.22 $ 3,158 Exercisable, September 30, 2019 2,161,959 $ 6.48 8.07 $ 2,505 Vested and expected to vest, September 30, 2019 3,207,803 $ 7.21 8.22 $ 3,158 Total stock-based compensation was as follows: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 (In thousands) (In thousands) Research and development $ 437 $ 477 $ 1,323 $ 774 General and administrative 771 794 2,114 1,544 Total $ 1,208 $ 1,271 $ 3,437 $ 2,318 Restricted Stock As of September 30, 2019, and December 31, 2018, there was $4,000 and $144,000, respectively, recorded in other long-term liabilities related to shares held by employees and directors that were subject to repurchase. A summary of restricted stock activity follows: Number of Restricted Shares Outstanding Unvested shares—December 31, 2018 49,052 Restricted shares vested (45,289 ) Unvested shares—September 30, 2019 3,763 Employee Share Purchase Plan (ESPP) In June 2018, the Company adopted the 2018 Employee Stock Purchase Plan (“ESPP”), which became effective upon the execution of the underwriting agreement related to the IPO. The initial offering period began June 27, 2018 and will end on August 15, 2020 with purchase dates of February 15, 2019, August 15, 2019, February 15, 2020, and August 15, 2020. Each subsequent offering will be approximately 24 months long and will consist of four purchase periods with purchase dates occurring on February 15 and August 15 of each year. On each purchase date, ESPP participants will purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the offering date or (2) the fair market value of the common stock on the purchase date. Total stock-based compensation related to the ESPP for the three and nine months ended September 30, 2019 was $175,000 and $592,000, respectively. Total stock-based compensation related to the ESPP for the three and nine months ended September 30, 2018 was $231,000. A total of 94,568 shares of common stock were purchased pursuant to the ESPP during the nine months ended September 30, 2019 for total proceeds of $860,000. Convertible preferred stock Through December 31, 2017, the Company’s convertible preferred stock was classified in permanent equity as redemption of the convertible preferred stock was in the control of the Company. In February 2018, the Company appointed an additional director which increased the influence of the convertible preferred stockholders on the board of directors. This change to the Company’s board composition resulted in the convertible preferred stock being reclassified outside of stockholders’ deficit because, in the event of certain “liquidation events” that were not solely within the Company’s control (including merger, acquisition, or sale of all or substantially all of the Company’s assets), the shares could become redeemable at the option of the holders. Upon the closing of the initial public offering on July 2, 2018, all outstanding shares of convertible preferred stock were automatically converted into 16,215,896 shares of common stock.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	9 . Net Loss Per Share The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: As of September 30, 2019 2018 Stock options to purchase common stock 3,207,803 3,345,155 Restricted stock subject to future vesting 3,763 60,342 Shares committed under ESPP 24,836 18,728 Total 3,236,402 3,424,225

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The interim condensed financial statements are unaudited. The unaudited interim condensed financial statements have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s financial position, results of operations and cash flows for the periods presented. The results of operations for the three and nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any other future annual or interim period. The condensed balance sheet as of December 31, 2018 included herein was derived from the audited financial statements as of that date. These condensed financial statements should be read in conjunction with the Company's audited financial statements and related notes as set forth in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018, as filed with the SEC on March 28, 2019.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenue and expenses during the reporting period. Significant estimates and assumptions made in the accompanying financial statements include but are not limited to revenue recognition, the fair value of stock options, the fair value of investments, income tax uncertainties, lease liability, and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates.
Revenue Recognition	Revenue Recognition License and Collaboration Arrangements The Company accounts for revenue recognition under Accounting Standards Codification (“ASC”), Revenue from Contracts with Customers (Topic 606) Under ASC 606, the Company recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services. The Company enters into collaborative arrangements with partners that fall under the scope of ASC, Collaborative Arrangements (Topic 808) In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues attributed to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not generally considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of any development milestones, and if necessary, adjusts its estimate of the transaction price. Any such adjustments would be recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment.
Foreign Currency Transactions	Foreign Currency Transactions Gains or losses from foreign currency transactions are included in interest and other income, net, in the statements of operations and comprehensive loss. The Company recognized $0.3 million in foreign currency gains for the three months and nine months ended September 30, 2019. Foreign currency losses for the three months ended September 30, 2018 were immaterial. The Company recognized $0.2 million in foreign currency losses for the nine months ended September 30, 2018.
Fair Value Measurement	Fair Value Measurement Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active; Level 3— Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.
Leases	Leases The Company adopted Accounting Standards Update No. 2016-02, Leases Rent expense for the operating leases is recognized on a straight-line basis over the lease term and is included in operating expenses on the statements of operations and comprehensive loss. Variable lease payments include lease operating expenses. The accompanying condensed financial statements as of and for the three and nine months ended September 30, 2019 are presented under ASC 842. The prior periods continue to be reported in accordance with previous lease guidance, ASC 840, Leases The impact of the adoption of ASC 842 on the accompanying condensed balance sheet as of January 1, 2019 was as follows: December 31, 2018 Adjustments due to the adoption of ASC 842 January 1, 2019 (In thousands) Assets Operating lease right-of-use asset $ — $ 2,328 $ 2,328 Liabilities and stockholders' equity Deferred rent classified as accrued liabilities $ 155 $ (155 ) $ — Lease liability, current $ — $ 968 $ 968 Lease liability, noncurrent $ — $ 1,847 $ 1,847 Deferred rent, noncurrent $ 331 $ (331 ) $ — The adjustments due to the adoption of ASC 842 related to the recognition of an operating lease right-of-use asset and lease liability for the Company's existing property operating lease and the derecognition of the deferred rent recognized under ASC 840. There was no impact on the Company’s statement of operations and comprehensive loss from the adoption and no cumulative-effect adjustment to the beginning accumulated deficit.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Schedule of Impact of Adoption of ASC 842 on Accompanying Condensed Balance Sheet	The impact of the adoption of ASC 842 on the accompanying condensed balance sheet as of January 1, 2019 was as follows: December 31, 2018 Adjustments due to the adoption of ASC 842 January 1, 2019 (In thousands) Assets Operating lease right-of-use asset $ — $ 2,328 $ 2,328 Liabilities and stockholders' equity Deferred rent classified as accrued liabilities $ 155 $ (155 ) $ — Lease liability, current $ — $ 968 $ 968 Lease liability, noncurrent $ — $ 1,847 $ 1,847 Deferred rent, noncurrent $ 331 $ (331 ) $ —

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis	The fair value and amortized cost of cash equivalents and available-for-sale securities by major security type as of September 30, 2019 and December 31, 2018 are presented in the following tables: As of September 30, 2019 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Market Value (In thousands) Money market funds Level 1 $ 30,546 $ — $ — $ 30,546 US government debt securities Level 1 28,039 13 (10 ) 28,042 Commercial paper Level 2 29,612 — — 29,612 Corporate debt securities Level 2 52,167 66 — 52,233 Asset-backed securities Level 2 25,051 22 (1 ) 25,072 Total cash equivalents and available-for-sale securities $ 165,415 $ 101 $ (11 ) $ 165,505 As of December 31, 2018 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Market Value (In thousands) Money market funds Level 1 $ 7,959 $ — $ — $ 7,959 US government debt securities Level 1 15,963 — (1 ) 15,962 Commercial paper Level 2 43,277 — — 43,277 Corporate debt securities Level 2 46,186 — (54 ) 46,132 Asset-backed securities Level 2 22,842 — (27 ) 22,815 Total cash equivalents and available-for-sale securities $ 136,227 $ — $ (82 ) $ 136,145
Schedule of Change in Fair Value of Embedded Derivative	The change in fair value of the embedded derivative is presented in the following table: Nine Months Ended September 30, 2019 (In thousands) Beginning balance $ 331 Change in fair value of embedded derivative (13 ) Ending balance $ 318

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Schedule of Accrued Liabilities	Accrued liabilities consist of the following: September 30, 2019 December 31, 2018 (In thousands) Accrued research and development expenses $ 12,605 $ 5,870 Accrued bonuses 2,119 1,602 Deferred rent, current — 155 Other 1,795 1,417 Total accrued liabilities $ 16,519 $ 9,044

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Schedule of Undiscounted Future Non-Cancellable Lease Payments under Company's Operating Leases	The undiscounted future non-cancellable lease payments under the Company's operating leases are as follows: September 30, 2019 (In thousands) Years Remaining 2019 $ 392 2020 1,584 2021 864 Total undiscounted lease payments 2,840 Present value adjustment for minimum lease commitments (178 ) Lease liabilities $ 2,662

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
Summary of Option Activity	The following summarizes option activity for the nine months ended September 30, 2019: Shares Issuable Under Options Weighted- Average Exercise Price Weighted- Average Remaining Contract Term Aggregate Intrinsic Value (In years) (In thousands) Balance, December 31, 2018 3,404,847 $ 6.55 8.82 $ 31,388 Options granted 233,652 13.55 Options exercised (316,084 ) 4.51 Options forfeited (114,612 ) 7.86 Balance outstanding September 30, 2019 3,207,803 $ 7.21 8.22 $ 3,158 Exercisable, September 30, 2019 2,161,959 $ 6.48 8.07 $ 2,505 Vested and expected to vest, September 30, 2019 3,207,803 $ 7.21 8.22 $ 3,158
Schedule of Total Stock-Based Compensation	Total stock-based compensation was as follows: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 (In thousands) (In thousands) Research and development $ 437 $ 477 $ 1,323 $ 774 General and administrative 771 794 2,114 1,544 Total $ 1,208 $ 1,271 $ 3,437 $ 2,318
Summary of Restricted Stock Activity	A summary of restricted stock activity follows: Number of Restricted Shares Outstanding Unvested shares—December 31, 2018 49,052 Restricted shares vested (45,289 ) Unvested shares—September 30, 2019 3,763

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Summary of Outstanding Potentially Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: As of September 30, 2019 2018 Stock options to purchase common stock 3,207,803 3,345,155 Restricted stock subject to future vesting 3,763 60,342 Shares committed under ESPP 24,836 18,728 Total 3,236,402 3,424,225

Description of Business - Addit

Description of Business - Additional Information (Details) - USD ($)	Jul. 12, 2019	Jul. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	[1]
Description Of Business [Line Items]
Cash, cash equivalents and short-term investments			$ 166,700,000
Cumulative net losses			201,154,000		$ 139,769,000
Issuance of common stock			859,000	$ 0
Net proceeds from offering			$ 80,461,000	$ 116,460,000
Maximum
Description Of Business [Line Items]
Securities available for issuance under shelf registration statement	$ 250,000,000
At-The-Market Offerings | Maximum
Description Of Business [Line Items]
Issuance of common stock	$ 60,000,000
Underwritten Public Offering
Description Of Business [Line Items]
Common stock shares, issued		10,781,250
Shares issued pursuant to exercise of underwriters' option to purchase		1,406,250
Common stock issued price per share		$ 8
Net proceeds from offering		$ 80,500,000
[1]	The balance sheet as of December 31, 2018 is derived from the audited financial statements as of that date.

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018
Accounting Policies [Abstract]
Foreign currency transaction gain (loss)	$ 0.3	$ 0.3	$ (0.2)

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Impact of Adoption of ASC 842 on Accompanying Condensed Balance Sheet (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Assets
Operating lease right-of-use asset		$ 2,282
Liabilities and stockholders' equity
Deferred rent classified as accrued liabilities				$ 155
Lease liability, current		1,434
Lease liability, noncurrent		$ 1,228
Deferred rent, noncurrent	[1]			$ 331
Accounting Standards Update 2016-02
Assets
Operating lease right-of-use asset			$ 2,328
Liabilities and stockholders' equity
Lease liability, current			968
Lease liability, noncurrent			1,847
Accounting Standards Update 2016-02 | Adjustments due to the adoption of ASC 842
Assets
Operating lease right-of-use asset			2,328
Liabilities and stockholders' equity
Deferred rent classified as accrued liabilities			(155)
Lease liability, current			968
Lease liability, noncurrent			1,847
Deferred rent, noncurrent			$ (331)
[1]	The balance sheet as of December 31, 2018 is derived from the audited financial statements as of that date.

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jul. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers between level 1 to Level 2 and Level 3 amount	$ 0		$ 0
Realized gains (losses) on investments	0	$ 0	0	$ 0
Leukemia & Lymphoma Society, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability						$ 0
Other Long-Term Liabilities | Leukemia & Lymphoma Society, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	$ 300,000		$ 300,000		$ 300,000
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity of investments			1 year		1 year

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value and Amortized Cost of Cash Equivalents and Available for Sale Securities by Major Security Type (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 165,415	$ 136,227
Unrealized Gains	101
Unrealized Losses	(11)	(82)
Market Value	165,505	136,145
Level 1
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	30,546	7,959
Market Value	30,546	7,959
US Government Debt Securities | Level 1
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	28,039	15,963
Unrealized Gains	13
Unrealized Losses	(10)	(1)
Market Value	28,042	15,962
Commercial Paper | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	29,612	43,277
Market Value	29,612	43,277
Corporate Debt Securities | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	52,167	46,186
Unrealized Gains	66
Unrealized Losses		(54)
Market Value	52,233	46,132
Asset-backed Securities | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	25,051	22,842
Unrealized Gains	22
Unrealized Losses	(1)	(27)
Market Value	$ 25,072	$ 22,815

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Change in Fair Value of Embedded Derivative (Details) - Embedded Derivative $ in Thousands	9 Months Ended
	Sep. 30, 2019USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of embedded derivative, Beginning balance	$ 331
Change in fair value of embedded derivative	(13)
Fair value of embedded derivative, Ending balance	$ 318

Balance Sheet Components - Sche

Balance Sheet Components - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accrued Liabilities Current [Abstract]
Accrued research and development expenses	$ 12,605	$ 5,870
Accrued bonuses	2,119	1,602
Deferred rent, current		155
Other	1,795	1,417
Total accrued liabilities	$ 16,519	$ 9,044	[1]
[1]	The balance sheet as of December 31, 2018 is derived from the audited financial statements as of that date.

Research and License Agreemen_2

Research and License Agreements - Additional Information (Details) ¥ in Billions	1 Months Ended			3 Months Ended		9 Months Ended			12 Months Ended
	Jul. 31, 2019USD ($)	Nov. 30, 2017USD ($)	Jan. 31, 2017USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2019JPY (¥)	Sep. 30, 2018USD ($)	Dec. 31, 2018USD ($)	Jul. 10, 2019JPY (¥)	Mar. 31, 2017USD ($)
Leukemia & Lymphoma Society, Inc. ("LLS")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable	$ 0
Derivative Liability	0
Leukemia & Lymphoma Society, Inc. ("LLS") | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional grant funding based on milestone payments	3,000,000
Potential future milestone payments related to certain development and regulatory approval milestones	6,000,000
Leukemia & Lymphoma Society, Inc. ("LLS") | Other Long-Term Liabilities
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Derivative Liability				$ 300,000		$ 300,000			$ 300,000
Research Grant | California Institute of Regenerative Medicine (CIRM)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable			$ 10,200,000
Collaborative arrangement research grant received				0		2,000,000
Collaborative arrangement milestone payments receivable period			4 years
Proceeds from collaborators						9,200,000			7,200,000
Research Grant | Leukemia & Lymphoma Society, Inc. ("LLS")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable											$ 4,200,000
Collaborative arrangement research grant received				300,000		300,000
Proceeds from collaborators						4,200,000			3,900,000
Research Grant | Leukemia & Lymphoma Society, Inc. ("LLS") | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments to be paid upon achievement of development and regulatory approval milestones				15,000,000		15,000,000
Research Grant | Leukemia & Lymphoma Society, Inc. ("LLS") | Other Long-Term Liabilities
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Derivative Liability				300,000		300,000			300,000
Research Grant | California Institute of Regenerative Medicine (CIRM) and Leukemia & Lymphoma Society, Inc. ("LLS")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants				600,000	$ 2,200,000	2,600,000		$ 5,600,000
Second Research Grant | California Institute of Regenerative Medicine (CIRM)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable		$ 5,000,000
Collaborative arrangement research grant received				500,000		1,200,000
Collaborative arrangement milestone payments receivable period		5 years
Proceeds from collaborators						3,600,000			2,400,000
Collaborative arrangement, reinstated research grant receivable consistent with original award amount	5,000,000
Collaborative arrangement, amended research grant receivable									$ 3,200,000
Collaborative arrangement, amended milestone payments receivable period									5 years
Amended March 2017 Agreement | Leukemia & Lymphoma Society, Inc. ("LLS")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payments received upon achievement of certain clinical or regulatory milestones						4,200,000
Research and Collaboration Agreements | Bluebird Bio, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants				1,000,000	$ 400,000	1,900,000		$ 1,000,000
Receivable for reimbursement of research and development costs incurred				$ 1,000,000		1,000,000
License and Collaboration Agreement | Ono Pharmaceutical Co., Ltd.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
One time upfront nonrefundable payment received						$ 15,700,000	¥ 1.7
Additional upfront payment receivable upon achievement of specified future development and commercial milestones | ¥										¥ 11.2
Collaboration arrangement, upfront fixed nonrefundable consideration	15,700,000
Collaboration arrangement, revenue recognized	$ 15,700,000

Related-Party Relationship wi_2

Related-Party Relationship with Stanford University - Additional Information (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Related Party Transaction [Line Items]
Payments for royalty		$ 0
License and Collaboration Agreement | Ono Pharmaceutical Co., Ltd.
Related Party Transaction [Line Items]
License payment received		15,700,000
Technology License Agreement | Stanford University
Related Party Transaction [Line Items]
Estimated sublicensing fee liability	$ 300,000	$ 300,000

Leases - Additional Information

Leases - Additional Information (Details)	1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 30, 2019USD ($)ft²	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)
Leases [Abstract]
Lease expiration date				Aug. 31, 2021
Renewal term		5 years		5 years
Tenant improvement allowance		$ 646,000		$ 646,000
Sublease agreement date	2019-04
Additional area rentable | ft²	6,230
Sublease expiration date	2021-02
Sublease amendment agreement date	2019-04
Additional space sublease commencement date	Mar. 1, 2021
Base rent per month	$ 36,000
Weighted average remaining lease term		1 year 9 months 18 days		1 year 9 months 18 days
Operating lease liabilities, weighted average incremental borrowing rate		7.00%		7.00%
Operating lease, liabilities, current		$ 1,434,000		$ 1,434,000
Operating lease, liabilities, noncurrent		1,228,000		1,228,000
Rent expense for operating leases		400,000	$ 200,000	900,000	$ 800,000
Variable lease payments for operating expenses		$ 200,000	$ 100,000	$ 400,000	$ 400,000

Leases - Schedule of Undiscount

Leases - Schedule of Undiscounted Future Non-Cancellable Lease Payments under Company's Operating Leases (Details) $ in Thousands	Sep. 30, 2019USD ($)
Operating Leases
Remaining 2019	$ 392
2020	1,584
2021	864
Total undiscounted lease payments	2,840
Present value adjustment for minimum lease commitments	(178)
Lease liabilities	$ 2,662

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Option Activity (Details) - 2015 and 2018 Stock Plan $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issuable Under Options, Beginning balance | shares	3,404,847
Shares Issuable Under Options, Options granted | shares	233,652
Shares Issuable Under Options, Options exercised | shares	(316,084)
Shares Issuable Under Options, Options forfeited | shares	(114,612)
Shares Issuable Under Options, Ending balance | shares	3,207,803	3,404,847
Shares Issuable Under Options, Exercisable | shares	2,161,959
Shares Issuable Under Options, Vested and expected to vest | shares	3,207,803
Weighted-Average Exercise Price, Beginning balance | $ / shares	$ 6.55
Weighted-Average Exercise Price, Options granted | $ / shares	13.55
Weighted-Average Exercise Price, Options exercised | $ / shares	4.51
Weighted-Average Exercise Price, Options forfeited | $ / shares	7.86
Weighted-Average Exercise Price, Ending balance | $ / shares	7.21	$ 6.55
Weighted-Average Exercise Price, Exercisable | $ / shares	6.48
Weighted-Average Exercise Price, Vested and expected to vest | $ / shares	$ 7.21
Weighted-Average Remaining Contract Term	8 years 2 months 19 days	8 years 9 months 25 days
Weighted-Average Remaining Contract Term Exercisable	8 years 25 days
Weighted-Average Remaining Contract Term Vested and expected to vest	8 years 2 months 19 days
Aggregate Intrinsic Value | $	$ 3,158	$ 31,388
Aggregate Intrinsic Value Exercisable | $	2,505
Aggregate Intrinsic Value Vested and expected to vest | $	$ 3,158

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Total Stock-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 1,208	$ 1,271	$ 3,437	$ 2,318
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	437	477	1,323	774
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 771	$ 794	$ 2,114	$ 1,544

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jul. 02, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 1,208,000	$ 1,271,000	$ 3,437,000	$ 2,318,000
Total proceeds			$ 1,426,000	166,000
Initial Public Offering
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Convertible preferred stock automatically converted into common stock						16,215,896
Employee Share Purchase Plan (ESPP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee share purchase plan description			The initial offering period began June 27, 2018 and will end on August 15, 2020 with purchase dates of February 15, 2019, August 15, 2019, February 15, 2020, and August 15, 2020. Each subsequent offering will be approximately 24 months long and will consist of four purchase periods with purchase dates occurring on February 15 and August 15 of each year. On each purchase date, ESPP participants will purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the offering date or (2) the fair market value of the common stock on the purchase date.
Employee share purchase plan purchase price percentage	85.00%		85.00%
Total stock-based compensation	$ 175,000	$ 231,000	$ 592,000	$ 231,000
Number of common stock purchase to ESPP			94,568
Total proceeds			$ 860,000
Restricted Stock | Other Long-term Liabilities
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares held by employees and directors subject to repurchase	$ 4,000		$ 4,000		$ 144,000

Stockholders' Equity - Summar_2

Stockholders' Equity - Summary of Restricted Stock Activity (Details)	9 Months Ended
	Sep. 30, 2019shares
Equity [Abstract]
Number of Restricted Shares Outstanding, Beginning balance	49,052
Number of Restricted Shares Outstanding, Restricted shares vested	(45,289)
Number of Restricted Shares Outstanding, Ending balance	3,763

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Outstanding Potentially Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	3,236,402	3,424,225
Stock Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	3,207,803	3,345,155
Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	3,763	60,342
Shares Committed under ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	24,836	18,728